x86: unregister PIT clocksource when PIT is disabled

The following scenario might leave PIT as a disfunctional clock source:

    PIT is registered as clocksource
    PM_TIMER is registered as clocksource and enables highres/dyntick mode
    PIT is switched to oneshot mode
    -> now the readout of PIT is bogus, but the user might select PIT
    via the sysfs override, which would break the box as the time
    readout is unusable.

Unregister the PIT clocksource when the PIT clock event device is switched
into shutdown / oneshot mode.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>